No need to drive yourself crazy looking for Key limes if they aren't local to you. This pie is great with regular lime juice, especially if you squeeze it fresh. Tip: roll the limes between your hand and a countertop to make them easier to squeeze.
Ingredients
For the Crust
1½ cups finely crushed graham cracker crumbs, from ~12 whole graham crackers
Preheat your oven to 375°F and set an oven rack in the middle position.
In a medium bowl, combine the graham cracker crumbs, brown sugar, and melted butter; stir with a fork first, and then your hands until the mixture is well-combined.
Using your fingers and the bottom of a glass or dry measuring cup, press the crumbs firmly into the bottom and up the sides of a 9 x 1.5-inch (deep-dish) pie pan.
The crust should be about ¼-inch thick. (Tip: do the sides first.)
Bake for 10 minutes, until just slightly browned. Let the crust cool on a wire rack.
While the crust cools, make the filling.
Lower your oven temperature to 350°F.
In a large bowl, whisk together the sweetened condensed milk, yogurt, lime zest, and lime juice.
Pour the thick mixture into the warm graham cracker crust. Bake for 15 minutes, until the filling is almost set. It should wobble a bit.
Let cool at room temperature for 30 minutes, then place in the refrigerator to chill thoroughly, about 3 hours.